The H-bridge stage, essentially made of four switches that are typically transistors of the bipolar or of the field effect type, is well known and widely used for driving reactive loads, as for example electric motors, solenoids or the like.
Driving in a swithing mode of such loads may occur according to an open-loop mode by employing for instance a pulse width modulation (PWM) circuit generating a periodic control signal with a variable duty-cycle for turning on/off the switches of the H-bridge. The duty-cycle of the PWM periodic signal is regulated for controlling the load. In practice, the circuit behaves as a generator of a clock or timing signal with a variable frequency so as to vary, for example, the rotation speed of a DC motor driven by the bridge.
In many applications, where at least during some phases of operation, it is necessary to control the load in a more precise and reliable way, a control feedback loop is realized by monitoring the current that flows through the load by the use of a sensing resistance connected in series to the bridge stage and comparing the voltage present on the sensing resistance by the use of one or more comparators. Typically, in a current-mode control loop, a first and second comparator are required. The first comparator is used for comparing the voltage on the sensing resistance with ground or virtual ground potential (zero crossing comparator). The second comparator is used for comparing the same voltage present on the sensing resistance with an adjustable reference voltage (sensing comparator).
In this operating mode, the turn-on fronts for the load-driving pair of switches can be provided by the same PWM circuit where such is present for implementing an open-loop mode of load control during different operating phases. By contrast, the turn-off fronts for the load-driving switches may be provided by the output of the sensing comparator.
As a matter of fact, very often the bridge control system is designed to alternatively implement an open-loop type of control or a closed-loop type of control.
This is the case for example of a DC motor used for advancing a paper ribbon through a printer. A fast page feed is implemented by driving the paper advancing motor in an open-loop mode, whereas the motor is driven in a closed-loop mode in order to exert a precise control of the line feed during printing. Of course, there are many other applications that take advantage of the possibility of commanding a control according to one or the other mode.
Often, when the system is functioning in a closed-loop mode, the switching noise generated by the bridge's switches can have an amplitude such as to cause spurious switchings. To avoid this drawback, it is a common practice to employ a number of RC filters or more generally analog lowpass filters to reject switching noise, generated at every change of state of the bridge's switches.
According to one of the many ways of driving a DC motor, the control of the four switches of a H-bridge stage implies turning on a first pair of switches during an excitation phase (Bridge.sub.-- ON), the turning on of the second pair of switches and the turning off of the first pair during a current recirculation phase (Bridge.sub.-- RIC) for discharging the energy stored in the motor winding inductance, and eventually the turning off of all the switches for setting the bridge in a so-called "tristate" condition (high impedance). Therefore, at every change of configuration of the bridge's switches, switching spikes that are eventually read by the control comparators may (if their amplitude and/or persistence is sufficient) cause false or improper switching commands. These disturbances are commonly filtered by lowpass filters having a time constant that can be adjusted in function of the type of motor or reactive load to be driven.
FIG. 1 is a basic scheme of the use of an RC lowpass filter realized by connecting external components to dedicated pins of the integrated device and to a ground node of the load supply circuit.
The use of lowpass analog filters (RC) requires at least a dedicated pin coinciding with the sensing and input node of each comparator. Of course, an alternative is that of realizing RC filter components in an integrated form within the device. This situation, apart from not allowing an adaptation of the integrated device to the specific impedance characteristics of the external load, requires a conspicuous area of integration.
On the other hand, logic filtering techniques of switching noise are commonly based on processing a certain input logic signal containing noises by means of a logic circuitry that comprises at least a delay network, necessary for producing an output logic signal coinciding with the input logic signal but free of noise. Even in this case it is necessary to employ RC delay networks or equivalents and furthermore these circuits are hardly adaptable to specific impedance characteristics of the load.